1. Field of the Invention
The present invention relates to separate and/or sequential solid-liquid separation stages used in mineral processing and refining. The present invention also relates generally to a slurry rheology modifiers or viscosity modifiers in the compaction zone of settled solids to reduce rake torque and increase underflow in countercurrent decantation vessels (CCD vessels) as well as non-CCD vessels. More specifically, the present invention relates to the use of slurry rheology modifiers to reduce slurry viscosity of the settled solids in CCD and other types of vessels. Further, the present invention relates to the addition of rheology or viscosity modifiers to a slurry to increase the rate of consolidation of the slurry.
2. Description of the Prior Art
Mineral processing and refining generally involves a number of separate and/or sequential solid-liquid separation stages for purposes such as recovering the liquid which contains valuable components, recycling of the liquid, elimination of solids from the liquid to permit additional processing of the liquid, recovery of solids from the liquid to permit additional processing of the solids and disposal of the solid waste.
For example, in the Bayer process for the production of alumina, bauxite ore is pulverized, slurried in water, and then digested with caustic at elevated temperatures and pressures. The caustic solution dissolves oxides of aluminum, forming an aqueous sodium aluminate solution. The caustic-insoluble constituents of bauxite ore (referred to as "red mud") are then separated from the aqueous phase containing the dissolved sodium aluminate. Solid alumina trihydrate product is precipitated out of the solution and collected as product.
In more detail, the pulverized bauxite ore is fed to a slurry mixer where a water slurry is prepared. The slurry makeup water is typically spent liquor (described below) and added caustic. This bauxite ore slurry is then diluted and passed through a digester or a series of digesters where alumina is released from the ore as caustic-soluble sodium aluminate. The digested slurry is then cooled to about 230.degree. F., typically employing a series of flash tanks wherein heat and condensate are recovered. The aluminate liquor leaving the flashing operation contains from about 1 to about 20 weight percent suspended solids, which solids consist of the insoluble residue that remains after, or is precipitated during, digestion. The coarser solid particles may be removed from the settler solid, normally a 15-30% solid slurry, is washed with on-coming liquor from the washing cycle, i.e., countercurrent displacement wash circuit or sand filter, whichever is being utilized. The resultant underflow, generally having from 10-40% solids, is subsequently washed a plurality of times. The wash waters containing suspended red mud and dissolved alumina and caustic are then also treated with chemicals to promote settling. The countercurrent washing circuit utilizes two or more washers which receive a mud washer feed slurry from either the settler underflow or other washer underflow, as well as any dilution liquor. As noted above, the red mud does not include any coarser particles removed prior to feeding the slurry to the primary or mud settler.
The at least partial separation of the red mud solids from the pregnant liquor at elevated temperatures by settling or by filtration is expedited by the use of a flocculant. This initial clarification of the pregnant liquor into a clarified liquor phase is referred to as the primary settler stage. Flocculating agents improve the separation of insolubles by increasing the rate at which the solids settle, by reducing the amount of residual solids suspended in the liquor, and by decreasing the amount of liquor in the settled solids phase.
Flocculation performance is highly important in the primary settlement stages. Red muds are comprised chiefly of iron oxides (at least about 50 weight percent of the red mud solids), together with silicon oxides, calcium oxides, sodium alumino-silicates, titanium oxides and other materials, and commonly represent from about 5 to about 50 percent of the dry weight of the bauxite ore. Generally, these muds are comprised of very fine particles, which hinder the desired rapid and clean separation of red mud particles from the solubilized alumina liquor. If the rate of separation is slow, output is materially diminished and overall process efficiency is impaired. If the separation is not clean, the resultant aluminate liquor will require a more extensive treatment to remove residual solids, and/or the alumina trihydrate recovered will contain levels of impurities that are undesirably high for many end-uses.
The polysaccharides starch and dextran were used early in red mud flocculation. For instance, U.S. Pat. No. 3,085,853, Apr. 16, 1963, Lesinski et al., uses native dextrans to increase the rate of sedimentation of finely divided solids in aqueous suspensions and thereby facilitate the separation of such solids. Later synthetic polymeric flocculants became more commonly employed for the Bayer process.
U.S. Pat. No. 3,390,959 issued Jul. 2, 1968 to Sibert, uses acrylate homopolymers and copolymers which contain not more than 20% of other ethylenically unsaturated polymerizable polar monomers for the Bayer process. Included in Sibert's polar comonomers are acrylamide and diethylvinylphosphonate, among others. Diethylvinylphosphonate is the diethyl ester of vinylphosphonic acid, and can be hydrolyzed to the monoethyl ester in caustic solution. Complete hydrolysis of diethylvinylphosphonate groups to dibasic vinylphosphonic acid salt groups is not observed in caustic solution under Bayer process conditions, so the polymers of Sibert are neither converted to in use nor are equivalent to the hydroxymethyl diphosphonic acid polymers of the present invention.
U.S. Pat. No. 3,397,953, Aug. 20, 1968, Galvin et al., uses a blend of starch and polyacrylic acid on red mud suspensions, noting that polyacrylic acid alone is not suitable as a flocculating agent. The polyacrylic acids exemplified generally have molecular weights of less than 300,000. The flocculation and sedimentation activity of the blend is exemplified in the primary settler stage of a bauxite process.
U.S. Pat. No. 3,445,187, May 20, 1969, Sibert, uses synthetic acrylic acid polymer alone to enhance the rate of separation of red mud solids from the aqueous caustic solutions during secondary clarification steps. The synthetic polymer used contains at least about 80 weight percent of the acrylic acid mer unit, and has a molecular weight in excess of 50,000, and preferably in excess of 100,000.
U.S. Pat. No. 3,541,009, Nov. 17, 1970, Arendt et al., uses a combination of causticized or modified starch, a water soluble polymer, and a caustic alkali to enhance the coagulation, sedimentation and/or filtration of aqueous suspensions of solids, including the settling of red mud from Bayer process liquor. The water soluble polymer is derived from at least one olefinically-unsaturated monomer and has a molecular weight in excess of 100,000.
U.S. Pat. No. 3,681,012, Aug. 1, 1972, Sibert, uses an acrylic acid polymer most preferably having a molecular weight of at least 1,000,000, either alone or in combination with starch, for clarification of digested bauxite containing solubilized alumina and red mud residues. U.S. Pat. No. 4,767,540, Aug. 30, 1988, Spitzer et al., uses a polymer that contains hydroxamic acid groups for the same purpose. U.S. Pat. No. 5,008,089, Apr. 16, 1991, Moody et al., uses a combination of dextran and synthetic anionic polymer for flocculating red mud in Bayer process liquors.
U.S. Pat. No. 5,217,620, Jun. 8, 1993, Mahoney et al., uses a combination of pullulan, lactan, rhamsan, or zooglan with a conventional water soluble anionic flocculant for red mud settling.
The synthetic flocculating agents employed for the settling or filtration of red mud are generally water soluble polymers of one or more ethylenically-unsaturated monomers, and have been used together, as noted above, with starch and dextran for aluminate liquor clarification. The synthetic flocculating agents are usually anionic, and the optimum anionic content of such polymer is usually related to the alkalinity of the liquor. In the washing circuit, the early wash liquors have the highest alkalinity and may require a more highly anionic polymer than the later wash liquors.
After the final wash step in the Bayer process, the mud is normally concentrated via vacuum filtration or other means to a plastic mass which must be transported and acted upon by chemical processing equipment such as pumps, conveyors, mixers and the like. The concentration of the red mud contained in these solid-like suspensions can range between 40-75%. The handling of such unworkable masses has always been a problem for the aluminum manufacturing industry, equipment designers and equipment manufacturers. If it were possible to readily decrease the viscosity of these materials so they could be acted upon by means of pumps and other fluid handling devices, a great improvement would be made in this art.
A number of other mineral refining and processing techniques involve the use of raked thickeners whereby a solid-liquid slurry is introduced into a vessel, permitted to settle and the thickened slurry disposed on the bottom of the vessel is at least partially removed by way of action of a rake which moves the thickened slurry towards an outlet where the slurry can be removed from the vessel by pumping. In practice, the rake drive and rake structure must be designed so that it can deliver the torque required to direct the thickened slurry towards the outlet. The torque required is a function of a number of variables, two of which are the viscosity and yield point of the thickened slurry or underflow. If the viscosity or yield point of the underflow could be reduced, the rake mechanism which includes the rake drive, rake arm and rake blade could be designed at a lower cost.
Further, if the viscosity and yield point of the underflow could be reduced, a higher slurry concentration could be provided without requiring an upgrade in the rake drive mechanism or structural strength of the rake. If the viscosity and yield point of the underflow could be reduced, the overall cost of raked thickeners could be reduced because the rake mechanism could be provided with a lighter structure, requiring fewer raking arms and/or a smaller drive unit. Still further, the efficiency of raked thickeners could be improved because the installation of larger rake blades would be possible for a more efficient sweeping action.
Unsuccessful attempts to reduce underflow rheology by the duel addition of flocculant and viscosity modifier to the feed of the vessel have previously been made.